A comparative study of the winged bean protease inhibitors and their interaction with proteases

Three protease inhibitors, namely a chymotrypsin inhibitor (WbCI), a trypsin inhibitor (WbTI) and a chymotrypsin trypsin inhibitor (WbCTI) were purified to homogeneity from the imbibed winged bean (Psophocarpus tetragonolobus (L.) Dc) seeds by affinity chromatography on trypsin and chymotrypsin columns. The novelty of the purification lies in exploiting their differential degrees of interaction with their cognate proteases, which results in relatively higher yields for the three inhibitors as compared to the other methods used for purifying any one of them. All have an apparent molecular mass of about 20 kDa as do members of the Kunitz family of protease inhibitors. The amino-terminal sequence of WbCTI is identical to the sequence of the WTI-1 reported earlier by Yamamoto et al. whereas WbTI is a new protein whose N-terminal sequence has no homology with the N-terminal sequence of any known proteins. A comparative study of all three inhibitors shows the presence of WbTI and WbCTI only in the seeds, whereas the presence of WbCI in other tissues has already been documented. Synthesis and degradation during germination follow a similar pattern for both the inhibitors. The binding studies with cognate proteases show that WbCTI inhibits trypsin more strongly than chymotrypsin and it never forms a ternary complex even though it binds both proteases. On the other hand, WbTI inhibited only trypsin, in spite of its binding to a chymotrypsin-Sepharose column. The study shows that these protease inhibitors are good candidates for studying the protein-protein interaction at the molecular level.